General error: 1215 Cannot add foreign key constraint - php

Question

I'm trying to get price value from products table and set it to sold table. Can u please tell me why I'm getting this error ? product_id and user_id works fine. But I get error when I need to create foreign price in sold table
SOLD table
Schema::create('sold', function (Blueprint $table) {
$table->increments('id');
$table->integer('user_id')->unsigned();
$table->foreign('user_id')->references('id')->on('users');
$table->integer('product_id')->unsigned();
$table->foreign('product_id')->references('id')->on('products');
$table->integer('price')->unsigned();
$table->foreign('price')->references('price')->on('products');
$table->integer('bid_price')->unsigned();
$table->foreign('bid_price')->references('bid_price')->on('products');
$table->timestamps();
});
PRODUCTS table
Schema::create('products', function (Blueprint $table) {
$table->increments('id');
$table->integer('user_id')->unsigned();
$table->foreign('user_id')->references('id')->on('users');
$table->string('slug');
$table->string('title');
$table->text('body');
$table->integer('price');
$table->integer('bid_price');
$table->string('address');
$table->string('condition');
$table->integer('quantity');
$table->string('img_1');
$table->string('img_2');
$table->string('img_3');
$table->string('img_4');
$table->integer('views');
$table->timestamps();
});
Error message:
General error: 1215 Cannot add foreign key constraint (SQL
: alter table `sold` add constraint sold_price_foreign foreign key (`price`
) references `products` (`price`))
EDIT
`LATEST FOREIGN KEY ERROR
------------------------
2016-03-01 12:40:08 31a0 Error in foreign key constraint of table auction/#sql-2564_ef:
foreign key (`price`) references `products` (`price`):
Cannot find an index in the referenced table where the
referenced columns appear as the first columns, or column types
in the table and the referenced table do not match for constraint.
Note that the internal storage type of ENUM and SET changed in
tables created with >= InnoDB-4.1.12, and such columns in old tables
cannot be referenced by such columns in new tables.
See http://dev.mysql.com/doc/refman/5.6/en/innodb-foreign-key-constraints.html
for correct foreign key definition.`

Answer 1

Cannot find an index in the referenced table
Your products table needs an INDEX on the column you are trying to reference. I'd say this has to be unique otherwise MySQL is unable to determine which row it is referencing.

Answer 2

Check this link: MySQL Cannot Add Foreign Key Constraint
The second answer there says:
I had set one field as "Unsigned" and other one not. Once I set both columns to Unsigned it worked.
So I would assume making the column "price" on your products unsigned too, should solve the problem.
Like this:
Schema::create('products', function (Blueprint $table) {
$table->increments('id');
$table->integer('user_id')->unsigned();
$table->foreign('user_id')->references('id')->on('users');
$table->string('slug');
$table->string('title');
$table->text('body');
$table->integer('price')->unsigned(); // <-- the difference is here!
$table->integer('bid_price');
$table->string('address');
$table->string('condition');
$table->integer('quantity');
$table->string('img_1');
$table->string('img_2');
$table->string('img_3');
$table->string('img_4');
$table->integer('views');
$table->timestamps();
});
Update:
The "LAST FOREIGN KEY ERROR" section tells you:
Cannot find an index in the referenced table
And the MySQL documentation says about foreign keys:
MySQL requires indexes on foreign keys and referenced keys
So basically you have to add an index to the price column in the products table.
I never worked with Laravel, but according to their documentation it should work like this:
$table->index('price');
Update 2:
I only considered the technical aspect here, but looking at the database structure in a semantical way, it is probably a bad idea to create a foreign key over price columns, because a price is not really an identifier for a product (products having the same price do exist!).